rst-malloc: s/rst_mem_cpos/rst_mem_align_cpos

Since we need to align some allocations (but not most of them), let's
always align them when checking the current position.

v2: always rst_mem_align() before the beginning of each "set" of
    allocations
v3: merge rst_mem_align and rst_mem_cpos

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
CC: Cyrill Gorcunov <gorcunov@gmail.com>
CC: Andrey Vagin <avagin@openvz.org>
Acked-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>